1. Technical Field of the Invention
The present invention relates to the transmission of video signals, and more particularly, to the transmission of video signals using line or frame synchronous frequency hopping.
2. Description of Related Art
The transmission of video signals involves the transmission of a video luminance signal between a transmitting and receiving unit. Creation of the video luminance signal is accomplished by vertical and horizontal raster scanning of the video images on a video image tube. The raster scanning includes horizontal line flyback times during which the video signal is blanked to the black condition by a line synchronization pulse while the scan returns back to the initial position for the next horizontal line. Vertical scan synchronization pulses are transmitted during a vertical frame flyback time wherein the scan returns from a lower right-hand to an upper or left-hand corner of a video frame.
FIG. 1 illustrates a prior art video wave form used for video transmission. A video signal 2 represents variations in image intensity over a period in time. The video signal 2 is separated into a plurality of line scan periods 4 which are separated by regular occurrences of line sync pulses 6. The video signal 2 is blanked to the black level during the line scan flyback time to prevent unwanted lines as the scan spot returns from right to left on the screen. For color video images, a chrominance signal (not shown) is added to the video signal. However, this does not materially alter the appearance of the video waveform depicted in FIG. 1.
After each 625 line period, a frame sync pulse 8 is inserted to indicate the separation between two video frames. The frame sync pulse 8 allows the frame scan to return to the top of the next video frame during the frame flyback time. The frame sync pulse synchronizes the vertical scan generator at the video signal receiver. The video signal is blanked to the black level during the frame flyback period to prevent generation of diagonal lines on the screen as the spot returns from the bottom right to the top left of the video frame.
When interlacing is used, every other horizontal line of the video frame is presented over two vertical scan periods. The vertical scan returns from the bottom right to the top left after approximately half the line periods and starts a new vertical scan to fill-in the alternate lines that were not included in the first half scan. Interlacing allows half of the horizontal lines to be updated twice as frequently, reducing image flicker and providing improved response to movement of the image. A complete image of 625 lines is thus transmitted every two vertical scan periods.
Conventional video broadcasts have used vestigial sideband amplitude modulation. This is a form of double-side band amplitude modulation in which the higher video frequency components of one sideband are suppressed by a filter to reduce transmission bandwidth requirements. The vestigial sideband transmissions are sensitive to interference. Thus, transmitters that are allocated the same channel frequency must be widely geographically spaced to prevent any interference.
Frequency modulation may alternatively be used to transmit the composite video signal in a manner that is much less sensitive to interference. However, frequency modulation requires a much wider bandwidth. FM video transmission is utilized in broadcast satellite/electronic news gathering services (BAS/ENG). The system utilizes an ad hoc channel allocation scheme whereby an ENG service wishing to temporarily setup within a given area applies for a frequency to a frequency allocating service.
The frequency allocating service allocates a frequency with regard to other users that may be close enough to create interference between signals. Individuals already utilizing the same frequency must be far enough away to cause no perceptible video picture degradation. The frequency allocating service must allocate frequencies on a conservative geographical spacing to guarantee non-interference. Conservative geographical spacing however reduces the efficiency of spectral use creating a poor utilization of resources. Therefore, a system enabling better utilization of video frequency transmission resources would be greatly beneficial.